Four alternative mechanisms for the benzenesulfonic acid + methanol esterification reaction have been studied at the B3LYP/aug-cc-pVTZ level. The participation of a pentacoordinate sulfur intermediate (in either neutral or protonated form) can be disregarded according to energy considerations. Instead, results show a low activation barrier for the S_N1 pathway (through a sulfonylium cation intermediate) and a moderate barrier for the S_N2 path (involving protonated methanol as an alkylating reagent).